Signaling circuit



H. NYQUIST SIGNALING CIRCUIT June 18, 1935.

Filed Jan. 24, 1953 FIG.

T l l FIG? FREDUFNCY FREQUENCY FREQUENCY INVENTOR H. NYQU/ST ATTORNEY Patented June 18, 1935 UNITED STATES PATENT OFFICE SIGNALING omcorr Application January 24,

g 6 Claims.

This invention relates to telegraph circuits and particularly to a method of and means for securing a satisfactory duplex balance between a fixed balancing network and each of a plurality 6 of circuits to be associated therewith, said circuits having various amounts of electrostatic capacity, which may be due, for example, to differences in the lengths of cables or subscribers loops contained in said circuits.

In telegraph exchange service, such as has been established for teletypewriter operation, each subscriber is connected to the nearest teletypewriter exchange by means of a cable or openwire circuit commonly known as a subscribers loop. Since subscribers are located at various distances from the exchange, the loops are of different lengths and have diiferentamounts of distributed capacity and resistance.

Whenever communication is established between two subscribers who are connected to a particular telegraph exchange, or between two subscribers connected to different telegraph exchanges, a cord circuit telegraph repeater is employed at each exchange. Such repeaters utilize the principle of duplex balance and employ fixed balancing networks which are required to give a satisfactory duplex balance when the repeater is connected to any subscribers loop, or to the local circuits of a toll telegraph repeater.

It has been found practicable heretofore to design a simple balancing network for the cord circuit repeater which is satisfactory for loop circuits having little or no capacity and for circuits having up to about 25 geographical miles of cable. For loop circuits containing longer lengths of cable, the duplex balance has been found unsatisfactory when using existing types of balancing networks, because the unbalance currents are of sufiicient magnitude to cause false operation of the receiving relay. Since teletypewriter exchange circuits are operated chiefly on a half duplex or to-and-fro basis, such false operation of a receiving relay will distort the signals which are 'beingtransmitted and in severe cases may cause a false break or interruption at the station from which the signals are being transmitted.

If the capacity of the balancing network were made suflicient to balance the large capacity of the longer subscribers loops, difiiculty due to unbalance currents would be experienced when the cord circuitrepeater isconnected to a loop circuit which happens to have only a small amountof capacity.

It is an object of this invention is to so modify 1933, Serial No. 653,326

the impedance characteristics of long loop circuits, as seen from the telegraph exchange, that the duplex balance will be improved and made satisfactory even with subscribers loop circuits whose lengths may be considerably greater than 25 miles.

It is a further object of this invention to reduce differences between the impedance charac-' teristic of the balancing network and that of a loop circuit having a long length of cable.

In pursuance of the objects stated the present invention contemplates the insertion of an ,inductance element, or of a network containing inductance, resistance and capacity in parallel, at a point located in proximity of the central exchange station. The effect of the insertion of such an element or network into a loop circuit upon the impedance characteristics of the loop circuit is to oppose the'variations with frequency of the impedance characteristics of the longer loop circuits, that is, the impedance characteristics of the loop circuits are modified to be substantially similar to the impedance characteristics of the balancing network, and the disad vantages referred to above are thereby avoided.

The invention will bebetter understood when read in connection with the accompanying drawing in which Figure 1 is a schematic diagram showing an embodiment of the invention; and

Figs. 2, 3 and 4 are diagrams illustrating operation of the arrangements of Fig. l.

In Fig. l the cord circuit repeater I!) is termi nated in two cords and plugs II and 20 which may be inserted in any two of the jacks 2 I, 21 or 33 in order to complete a circuit for telegraph operation. If the plug l! is inserted in jack 2| and the plug 20 in jack 33, signals originating at a subscribers key orteletypewriter pending'contacts 4| will be transmitted as open and close signals over the cable 40, network 39, jack 33 through the cord circuit repeater Ill, jack 2|, toll telegraph repeater 25 and over the toll line 26 to the distant subscriber 5|. Alternately, signals transmitted by the distant subscriber 5! and con-' ducted over the toll line 26 pass in the opposite direction over the circuit traced above and operate'the local subscriber's sounder or teletypewritor receiving mechanism 42. This sounder or receiving mechanism is also operated when signals are sent from the key 4| and thereby furnishes to the local subscriber a copy of the signals which he transmits. Telegraph operation over such circuit arrangements is necessarily on a half-duplex basis, that is, signals maybe transmitted in ei-' the ther direction but in only one direction at a time.

The cord circuit repeater I0 consists of two polar relays I2 and I6, each having two windings which are difierentially connected in series with each other. The upper winding of each relay is connected to plugs I I and 20 respectively for connection to the circuit over which communication is to be established, while the lower winding of each relay is connected to fixed balancing networks I3 and II, respectively. The armature 5 of each relay is connected to the common connec tion of two windings of the other relay; the marking contacts M of the two relays are connected to the positive terminals of marking batteries I4 and I8 respectively; the spacing contacts S are connected to the negative terminals of batteries I5 and I9 respectively. Signals received over the cord circuit and plug connected to either of the relays I2 and I6 cause the armature of that relay to operate between its marking and spacing contacts and thereby transmitthe signals in polar form to the cord and plug and to the balancing network connected to the other relay. Since the windings of each relay are connected differentially with respect to the signals repeated by the other relay, the repeated signals will not operate the relay through whose windings they are transmitted, provided the impedance of the balancing network which is connected to one winding is sufliciently like that of the loop circuit which is connected to the other winding. Such balanced circuit arrangements are well known in the art since they are frequently employed in duplex telegraph operation over D. C. circuits.

The operation of the circuit arrangements shown in Fig. 1 will be understood from the following description. Assume that plug I I is connected to jack 21 and the plug 20 to jack 33. When the circuit is idle or in the marking condition, currents of about 20 milliamperes will flow, over each of the two circuits 30 and 40. In circuit 30 the current will be due to the grounded negative battery 29 in series with the grounded positive battery I8 which is connected to the marking contact M of the relay I 6. This current flows through the windings of the sounder or printer magnet 32 and also through one winding of relay I2 and holds the armature of the sounder in the marking condition and that of the relay I2 on its marking contact M. The battery I8 also causes a current of about 10 milliamperes to flow through the other winding of relay I2 to the balancing network I3 to ground which tends to operate the armature of this relay to its spacing contact S. Similarly the armature of relay I6 is held on its marking contact M by a current of 20 milliam- 'peres flowing from the batteries 35 and I4 over the circuit 40 and through one winding of the relay I6, while a current of 10 milliamperes flows from battery I4 in an opposite direction through the other winding of the relay and through the balancing network I! to ground.

When signals are transmitted by means of any of the contacts 3 I, circuit 30 is closed to transmit a marking signal and opened to transmit a spacing signal. In the case of a marking signal the circuit conditions are as described above. In the case of a spacing signal the current of 20 milliamperes in the circuit 30 is reduced to zero by the opening of the contacts 3|, and the sounder or printer magnet 32 operates to spacing under the influence of its spring tension while the relay I2 operates its armature to the spacing contact S under the influence of the 10 milliamperes of current flowing through one of its windings and the balancing network I3. The operation of relay I2 to its spacing contacts connects the negative battery I5 in place of the positive battery I4 to the windings of relay I6 and thereby to the loop circuit 40 and the balancing network II. The battery I5 opposes the battery 35 thereby reducing the current in circuit 40 to substantially zero and operating the sounder 42 to spacing in accordance with the received signal. The armature of relay I6 will tend to remain on its marking contact M since the 10 milliamper-es of current flowing from I the negative battery I5 through one winding of the relay and through the balancing network II to ground will be in a direction to holdthe relay armature on the marking contact.

If the circuit 40 has considerable capacity to ground which is not sufiiciently balanced by the capacity in the balancing network I 1, the temporary charging current flowing through the upper winding of relay I6 and into circuit 40, when relay I2 operates to its spacing contacts, may cause a temporary operation of the armature of relay IB from its marking contact M to its spacing contactS. Such a false operation of relay I6 would transmit short spacing signals into the circuit 30 and interfere with the transmission of signals from that circuit. If the capacity of the balancing network II were made sufficient to balance the large capacity'of circuit 40, difficulty would be experienced when the cord and plug 20 of the cord circuit repeater is connected to a loop circuit which happens to have only a small amount of capacity. In the latter case the armature of relay I6 would operate temporarily to its spacing contact whenever the armature of relay I2 operates from its spacing contact to its marking contact, because 'the excess charging current taken by the balancingnetwork I! would then be in a direction to operate relay I6 to spacing.

The false relay operations described above may, in general, be considered to be due to an appreciable difierence between the impedance characteristics of the loop circuit and of the balancing network in the frequency range over which the relay responds. Fig. 2 is a diagram showing the components of the impedance characteristic of the balancing network I3 or IT, while Fig. 3 is a similar diagram of the components of the impedance characteristic of a loop circuit containing a long length of cable. In both of these figures as well as in Fig. 4, R is the resistance component and X the reactance component of the impedance. It is evident from an inspection of Figs. 2 and 3 that there is an appreciable difference between the impedance characteristic of the network I3 or IT and that of a loop circuit containing a long length of cable. For cables up to about 25 miles in length the resistance element 28 or 34, which is usually inserted in the loop circuits at the telegraph exchange to equalize the direct current resistance of the loops, tends to prevent large differences between the impedance characteristics of the loop circuit and the balancing network. For loop circuits containing cables longer than about 25 miles, the value of the equalizing resistance becomes too small to prevent an excessive difference in impedance between the loop circuit and the balancing network, since with long loops the surge of charging current into the line is greater than that into the balancing network. This causes the relays I 2 and I 3 of the cord circuit repeater to be frequently falsely operated during the transmission of signals over long loop circuits, and thus prevents satisfactory communication over such circuits.

The above mentioned disadvantages are avoided in accordance with the principles of this invention by substantially preventing or at least greatly reducing differences between the impedance characteristics of the balancing network and those of loop circuits having long lengths of cable. This is accomplished by a special network containing inductance which is connected in series in the loop circuit. Such a network is shown at 39 in Fig. 1; it comprises the in..- ductance element 36, the capacity element 3'! and the resistance element 38, all connected in parallel. This network may preferably be connected into the subscribers loop circuit 40 at the telegraph exchange and may either entirely replace the equalizing resistance 34, or it may be added to that resistance and connected in series with it.

Fig. 4 shows the eifect of the insertion'of such a network into a loop circuit upon the impedance characteristics of the circuit as seen from the jack 33 at the telegraph exchange. A comparison of Figs. 2, 3 and 4 shows that, whereas the impedance characteristic of Fig. 3 departs greatly from that of Fig. 2, the impedance characteristic of Fig. 4 differs only to a minor extent from that of Fig. 2, since the capacity reactance X of the cable is neutralized to a considerable extent. If, therefore, the longer loop circuits are equipped with a network 39 according to the invention, a fixed balancing network, such as l3 or IT, may be utilized in the cord circuit repeater to balance practically all the subscribers loops which may be encountered in practice. The values of the elements 36, 31 and 38 of the network 39 are dependent upon the characteristics of the balancing network l3 or H and upon the characteristics of the loop circuit 40. These values are readily determined from experiments by sending signals over the cord circuit repeater and into the loop circuit 40, and selecting such values for the elements that the impedance looking into the cable when the network is in place, more nearly approaches that of the balancing network or artificial line of the repeater.

In a specific practical embodiment of this invention, satisfactory operation of a loop circuit consisting of a 19 gauge conductor of about 32 miles in length may be obtained by replacing the equalizing resistance 34 (which may have a value of say 700 ohms) by a parallel network containing an inductance element 36 of about 6 henries, a condenser 31 having a capacity of about microfarad, and a resistance 38 of about 1500 ohms.

In United States Patent 1,959,412, granted May 22, 1934, to A. D. Dowd, there has been disclosed and claimed a method of and means for improving the efficiency of telegraph cable loop circuits whereby the length of subscribers loops may be increased, by inserting a current regulating resistance in the proximity of the subscribers station. The arrangements disclosed in the present application may be employed in conjunction with those described in the Dowd application and the results obtained under certain circumstances, at least, from such combined use are such that satisfactory operation of longer loop circuits is obtained than would result from the single use of only one of the arrangements proposed in the two applications.

While the invention has been described as embodied in a certain specific form, it is understood that it is capable of embodiment in other and varied forms without departing from the spirit of the invention as defined by the appended claims.

What is claimed is:

1. In a telegraph system, a plurality of subscribers lines, a telegraph repeater having a fixed balancing network for balancing the impedance characteristics of each of said plurality of lines with which said repeater may be connected, means connected to said lines for neutralizing the capacity reactance of said lines, said means comprising an electrical network containing an inductance element.

2. In combination, a plurality of telegraph circuits having various impedance characteristics, a telegraph repeater for repeating signals between said circuits, said repeater having a fixed balancing network for balancing said telegraph circuits, and electrical networks connected in at least certain of said telegraph circuits, said networks containing inductance, capacity and resistance elements connected in parallel and being adapted to modify the impedance characteristics of said telegraph circuits so as to render it substantially similar to the impedance characteristics of said balancing network.

3. In a telegraph exchange system, a direct current equalizing resistance, a telegraph repeater, a plurality of telegraph circuits of different impedance characteristics, means for connecting said repeater to said telegraph circuits for repeat* ing signals between said circuits, a balancing network for said repeater, said balancing network being capable of balancing the impedance characteristics of a substantial portion of said plurality of circuits, and an electrical network connected in each of the remainder of said plurality of circuits, said electrical network containing an inductance element and being adapted to modify the impedance characteristics of the remainder of said circuits so as to be capable of being balanced by said balancing network.

4. In a telegraph switching arrangement, the method of securing a balance between a fixed balancing network in a telegraph repeater and a plurality of telegraph circuits of different lengths with which the repeater may be associated, which consists in adapting said balancing network to balance the shorter of said telegraph circuits, and inserting electrical networks in the remainder of said telegraph circuits, said networks containing essentially capacity, inductance and resistance elements in parallel connection, whereby the impedance characteristics of the remainder of said telegraph circuits are so modified as to be substantially balanced by the impedance characteristics of said fixed balancing network.

5. Means for changing the terminal impedance characteristics of each line of a group of long direct current telegraph lines so that the terminal impedances of all of the lines tend to be substantially the same and tend to be independent of the lengths of said lines which comprises connecting a network connected directly in series with each of said lines, said network including a resistance and an inductance connected in parallel.

6. A-plurality of networks, one of said networks being individual to and connected directly in series with each of a group of long direct current telegraph lines of different lengths to reduce the variations in the terminal impedance characteristics of the different lines of different lengths, said network comprising a resistance, an inductance, and a capacitance connected in parallel.

HARRY NYQUIST. 

